As an ecologist and biodiversity researcher and recorder, the author visits a wide range of rural and urban habitats mainly close to his home in Sedlescombe near Hastings, East Sussex, UK. The weblog covers the full spectrum of wildlife, from mammals to microbes. As well as details of encounters with England’s flora and fauna, information on where to see species of interest is often given.

Sunday, March 12, 2017

Behaviour of Eudasyphora cyanella, the green cluster fly

Eudasyphora cyanella, a muscid sometimes known as the green cluster fly, emerges from hibernation at this time of year. I noticed twenty or thirty of them yesterday settling on shiny sunlit leaves of cherry-laurel in our lane. They did not seem to be interested in other leaves and, if disturbed, quickly returned.

I think the attraction might be that they think the leaves are wet and they might be thirsty. Some years back I made, in our garden, an artificial tree hole in a bucket filled with water and dead wood over which I hung a sleeve cage so that insects could get in via the lower open end.

I wrote the below at the time:

On 18th April 2000, a cold, grey day with heavy
showers there were, unexpectedly, seventeen of the distinctive muscid
greenbottle Eudasyphora cyanella (Meigen), all female, in the trap. Several females of this species had occurred
during the autumn, but none had been recorded since then. E.
cyanella breeds in fresh cow-dung and larvae have also
been recorded from sheep dung .
Both cattle and sheep graze regularly in pastures about 100 metres from
the trap, but the insects would have had to fly through woodland and scrub to
reach the rot-hole.

During the next few weeks numbers of E. cyanella rose steadily, though there were several days when
none, or few appeared. Females were
quite quickly joined by males and, on the 25th April, there were at
least 200 examples in the trap showing a marked clustering behaviour as they
gathered on the ‘roof’. Though they were
extremely active, no mating was observed.
During this period several other species of Diptera were, as usual,
found in the trap but in nowhere near the quantity of E. cyanella.

On 24th April, a reasonably sunny but quite cool
day, I spent some time watching the trap area in the early afternoon. E.
cyanella were sunning themselves on leaves at ground level near the trap
and some would rest on the side of the bucket. On one occasion one was observed on the rim
of the bucket apparently drinking from the moisture there and on one of the
projecting pieces of wood (these had by now absorbed much water). On leaving they tended to fly up into the
hanging sleeve. The flies seemed to make
little distinction between warm, sunny days and cool, cloudy ones, but numbers
were much lower during or after rain, the inference being that there was plenty
of free moisture available to drink from vegetation.

By 18th May the numbers of E. cyanella had fallen to virtually nil, but the species was
frequently observed in the wider countryside and singletons turned up in the
trap until mid-August.

One of the characteristics of Eudasyphora is that, unlike nearly all other Muscids, the adults
hibernate in winter.
One of the possible explanations of the observed behaviour was that,
after their winter rest, they needed to drink, though why they should be
particularly attracted to a rot-hole at a period when plenty of alternative
sources of water were available is not clear.
Whatever the reason for their behaviour, it had some further ecological
consequences. They produced, for
example, a substantial amount of excrement much of which must have finished up
in the rot-hole, thus significantly increasing the nutrient-level of a habitat
which is normally nutrient-poor. At this
time the trap also attracted a considerable swarm of the biting midge Culicoides obsoletus (Meigen). They
would settle in some numbers on the black netting and in rather smaller
quantities on my arms and face. They bit
occasionally, but their main interest seemed to be the trap. On looking carefully at these midges, they
seemed to be more interested in the droppings of the Muscids (which adhered to
the netting) than the flies themselves, though I did see them settle on some of
the larger species as though to bite. C. obsoletus breeds in a wide range of
moist decaying matter and it has been reported from both rot-holes and sheep dung. It
was not, however, biting in other parts of the neighbourhood and would
therefore appear to have been attracted by E.
cyanella and the other flies, rather than by myself.